The present invention is related to a high frequency transmission line. More specifically, the present invention is directed to a high frequency transmission line which is fabricated by combining different high frequency transmission lines with each other, and also directed to a high frequency board on which these different high frequency transmission lines are wired.
Conventionally, either microstrip lines or triplate lines are known as high frequency transmission lines for transmitting the high frequency signals. Generally speaking, as indicated in FIG. 14, a microstrip line is constituted by a dielectric substrate 2, a ground conductor (ground layer) 1, and a signal line (signal conductive line) 3. The ground conductor 1 has been formed on a lower plane (namely, lower layer) of this dielectric substrate 2. The signal line 3 has been formed on an upper plane (namely, upper layer) of this dielectric substrate 2. It should be noted that FIG. 14 shows a sectional view in the case that the microstrip line is viewed from a sectional plane perpendicular to the direction of the signal line 3. In the microstrip line shown in this example, dimensions and the like of the respective conductors have been set to the following values: For instance, a thickness of the dielectric substrate 2 is set to 320 μm; a width of the signal line 3 is set to 500 μm; and a characteristic impedance is set to 50 ohms.
Also, as indicated in FIG. 15, a triplate line is constituted by a dielectric substrate 2, ground conductors 1, and a signal line (signal conductive line) 4. The ground conductors 1 have been formed on both an upper plane and a lower plane (namely, both upper layer and lower layer) of this dielectric substrate 2. The signal line 4 has been embedded in an inner layer of the dielectric substrate 2 in such a manner that this signal line 4 is sandwiched by the upper and lower ground conductors 1. It should be noted that FIG. 15 also shows a sectional view in the case that the triplate line is viewed from a sectional plane perpendicular to the direction of the signal line 4. In the triplate line shown in this example, dimensions and the like of the respective conductors have been set to the following values: For instance, a thickness of the dielectric substrate 2 is set to 320 μm; a width of the signal line 4 is set to 115 μm; and a characteristic impedance is set to 50 ohms.
The above-described microstrip line, or the above-explained triplate line transmits high frequency signals having frequencies of e.g., 20 to 30 GHz along a direction of either the signal line 3 or the signal line 4 by electromagnetically coupling the signal line 3 to the ground conductor 1, or by electromagnetically coupling the signal line 4 to the ground conductors 1.
On the other hand, in the case that a high frequency signal is transmitted by employing both a microstrip line and a triplate line, for example, in such a case that in a high frequency board, such a high frequency signal received by an antenna which has been formed on a front surface of the high frequency board is conducted to a high frequency circuit which has been formed inside the high frequency board, or on a rear surface thereof, as represented in FIG. 16, the signal line 3 of the microstrip line is simply connected to the signal line 4 of the triplate line by way of a via hole conductor 6. Then, the high frequency signal is transmitted through such a high frequency transmission line which has been formed by connecting these signal lines 3 and 4 to each other (note that FIG. 16 is sectional view in such case that area located in vicinity of connection portion between both signal lines 3 and 4 is viewed from sectional plane located parallel to signal line direction). However, in such a connection structure, signal reflections occurred at the connection portion are increased with respect to the above-described high frequency signals, so that the high frequency signals can hardly pass through this connection portion.
On the other hands, there is one public knowledge example (for example, refer to patent publication 1) which indicates a connection structure capable of reducing signal reflections which are produced in a connection portion between high frequency transmission lines having two same constructions. However, this known connection structure cannot be directly applied to a connection portion between two high frequency transmission lines having different structures from each other.
Also, there are other public knowledge examples (for instance, refer to patent publications 2 and 3) capable of reducing signal reflections which are produced at a connection portion of high frequency transmission lines having different structures. However, any of methods disclosed in these public knowledge examples is realized based upon such an initial condition that the respective signal lines are present on the same plane. As previously explained, these public knowledge methods cannot be applied to such a case that the signal lines are arranged in different dielectric substrate layers.
[Patent Publication 1]
Japanese Laid-open Patent Application No. 2000-114801
[Patent Publication 2]
Japanese Laid-open Patent Application No. 2000-068715
[Patent Publication 3]
Japanese Laid-open Patent Application No. Hei-9-321507
As explained above, in such a structure that the microstrip line is simply connected to the triplate line by way of the via hole conductor 6, since there is a difference between the signal line widths of the transmission lines, a deviation of impedances is produced in the connection portion. This impedance deviation does not cause a serious problem in such a case that a frequency of a transmission signal is low. However, when a frequency of a transmission signal is high, this impedance deviation cannot be neglected. As a result, an impedance matching condition can be no longer established in the connection portion, and thus, reflections of transmission signals may occur, which may cause a signal passage characteristic to be deteriorated.